# Electron spin

1. May 9, 2009

### physicsnewb7

Can electron spin change? If so how does this happen?

2. May 10, 2009

### granpa

the direction of its spin will precess around an externally applied magnetic field. also it can change from diamagnetic to paramagnetic.

3. May 10, 2009

### meopemuk

When people talk about spin then may mean two different things. One is the absolute value of spin (the length of the vector). For the electron this value is $$\hbar/2$$, and it never changes, i.e., this is a fixed property of the electron, like its mass or charge.

Another thing is spin projection on a given axis (a vector component). This projection may be either $$+\hbar/2$$ or $$-\hbar/2$$, with probability weight assigned to each value. These probabilities may change in electron interactions, collisions, etc.

4. May 10, 2009

Staff Emeritus
It is correct that the orientation of the electron spin can be changed.

It is incorrect to describe a single electron as paramagnetic or diamagnetic. These are properties of bulk materials, not individual electrons.

5. May 10, 2009

### granpa

well yes. I was speaking loosely. but the electron will either align with or against the applied magnetic field. this is analogous to para and diamagnetism.

6. May 11, 2009

Staff Emeritus
Grampa, for heaven's sake, please turn your fount of misinformation down a notch.

First, as I said before, it is incorrect to describe a single electron as paramagnetic or diamagnetic. These are properties of bulk materials, not individual electrons. Second, the only person discussing applied magnetic fields is you. Third, the terms describing the orientation of spins with respect to external fields is not para- and dia-, but rather para- and ortho-.

7. May 11, 2009

### Bob S

The electron cannot change the magnitude of its spin or its magnetic moment. In the hydrogen atom, it is in the field of the proton spin (much weaker). There are only two alignments permitted; same direction, and opposite direction, They differ by 1420 MHz (21 cm). This is perhaps the most dominant microwave emission (and absorption) line in the universe.

8. May 11, 2009

### physicsnewb7

So the magnitude of the vector doesn't change but it's components do in a conservative way so as to keep a constant spin magnitude of h/4pi.

9. May 11, 2009

### meopemuk

That's right.

10. May 11, 2009

### sokrates

Pure physicists may not be aware of it, but the only real practical manifestation of spins is exposed by applying some equivalent of an external magnetic field to the device.

That basically takes spin (and all the entailing theoretical discussion) out of the Hilbert space and shows that it's real and it could be used.

The fact that Grampa's referring to - precession of spin - is the basis of the first proposed spinFET in 1989 by Datta and Das.

In fact, spin and magnetic fields are so entangled that the entire field of Spintronics (crowned by its first Nobel prize in 2007) is founded upon those two.

I'd go easy with Grampa if you haven't read a sentence involving spins and applied magnetic fields. Because that's your fault.

Last edited: May 11, 2009
11. May 11, 2009

### sokrates

This must be a sticky post to every spin question. first understand what you are talking about:

i) Is it the spin vector (projection)?
ii) Or is it simply the magnitude of that?

The first one will be important when magnetization and EXTERNAL magnetic fields are present

12. May 12, 2009

Staff Emeritus
Isn't it $\sqrt{3}\hbar/2$? It's actually in an eigenstate of S2, right?

13. May 12, 2009

### Matterwave

No, the electron's spin is $$\frac{\hbar}{2}$$

It is in an eigenstate of $$S^2$$ with eigenvalue $$\frac{\hbar^2}{4}$$. Take the square root of that and you get the correct answer.

14. May 12, 2009

Staff Emeritus
Why isn't the eigenvalue $\sqrt{S(S+1)}$?

15. May 12, 2009

### Staff: Mentor

No, the "z-component" (actually the component along any direction) of the spin angular momentum vector has that value (either + or -).

The magnitude of the spin angular momentum vector is a fixed $\sqrt{3} \hbar / 2$.

http://hyperphysics.phy-astr.gsu.edu/Hbase/spin.html

16. May 12, 2009

### Matterwave

Ah, you are indeed correct. I was wrong. Sorry.

Indeed, I had forgotten that the spin can never point directly in the +/- z direction.

17. May 12, 2009

### granpa

so what are the allowed z-component values for a delta baryon? (spin 3/2)

18. May 12, 2009

### Matterwave

3/2, 1/2, -1/2, -3/2

19. May 12, 2009

### granpa

thank you. :-)

20. May 12, 2009

### Staff: Mentor

Don't forget to multiply by $\hbar$ if you're talking about the physical quantity (angular momentum) and not the quantum number.